In order to prevent a light emitting element such as a semiconductor laser from causing a defective operation by heat generation of itself, it has been necessary that a semiconductor element mounting substrate (a heat sink, a heat transferring substrate, a housing, or the like) to which the light emitting element is connected is excellent in heat transfer characteristics, and conventionally, the semiconductor element mounting substrate has been generally formed of a ceramic such as AlN or SiC, that is high in thermal conductivity and has a favorable heat transfer characteristics. However, with a higher power output by a light emitting element in recent years, a higher degree of heat transfer characteristics has come to be demanded in a semiconductor element mounting substrate than what it currently has.
Therefore, in order to meet the above-mentioned demand, it was proposed to form the semiconductor element mounting substrate using a diamond composite material in which a number of fine diamond particles are bonded by, for example, a metal such as Cu or Ag or a ceramic such as SiC. This was based on a consideration that since diamond has the highest thermal conductivity among all the substances, by using the diamond composite material to form a semiconductor element mounting material, the thermal conductivity thereof can be dramatically improved in comparison with that of a conventional one formed of a ceramic or the like.
In order to conduct heat as smoothly as possible between a semiconductor element mounting substrate formed of a diamond composite material and a semiconductor element such as a light emitting element to be connected (mounted) to the semiconductor element mounting substrate or another heat transferring member thermally connected to a semiconductor element mounting substrate and assisting in heat transfer from a semiconductor element (hereinafter, these are sometimes generally called “another member”, it is necessary that the semiconductor element mounting substrate and another member are connected, via a layer of a solder or brazing material, in a closely adhered state with as small a gap as possible therebetween.
And, for this, it is necessary that a connecting surface, connected with another member, of a semiconductor element mounting substrate formed of a diamond composite material, such as an element mounting surface to be connected with a semiconductor element or a heat transfer surface to thermally connect another heat transferring member to the semiconductor element mounting substrate, is finished as smooth as possible. Therefore, for finishing the connecting surface of the semiconductor element mounting substrate smooth, the connecting surface has been conventionally polished by use of a diamond whetstone or the like. Moreover, the polished connecting surface has been coated with a metal film.
For example, in Patent Document 1, it is described that a semiconductor laser mounting sub-carrier serving as a semiconductor element mounting substrate is formed of the diamond composite material, that a connecting surface of the sub-carrier is polished such that an arithmetic mean roughness Ra of a roughness curve, showing a surface roughness, prescribed in Japanese Industrial Standards JIS B 0601:2001 “Geometrical Product Specification (GPS)—Surface Texture: Profile method—Terms, definitions and surface texture parameters,” becomes Ra≦0.5 μm, and that the polished connecting surface is coated with a first metal film made of at least one type of metal selected from a group consisting of Ni, Cr, Ti and Ta and a second metal film made of at least one type of metal selected from a group consisting of Mo, Pt, Au, Ag, Sn, Pd, Ge and In, in this order.
Moreover, it is described in Patent Documents 2 and 3, respectively, that a heat sink serving as a semiconductor element mounting substrate is formed of the diamond composite material and a connecting surface of the heat sink is polished so that an arithmetic mean roughness Ra becomes 0.2 μm or less in Patent Document 2 and 0.5 μm or less in Patent Document 3, and that the polished connecting surface is coated with the same first and second metal films as in the above.
However, since it takes a long time to polish a connecting surface of a semiconductor element mounting substrate formed of a diamond composite material by use of a diamond whetstone or the like until it reaches the predetermined surface roughness, so that there is a problem that productivity of a semiconductor element mounting substrate declines and the manufacturing cost rises. When the connecting surface is polished up to the predetermined surface roughness by flat polishing using, for example, a #100 to #400 diamond whetstone, it takes approximately 20 hours or more to finish one connecting surface.
In addition, even if a connecting surface could be polished to the predetermined surface roughness, since a large number of recesses produced by diamond particles dropped at polishing and protrusions caused by diamond particles remaining unpolished exist on the connecting surface, unevenness formed on the connecting surface by the recesses and the protrusions hinders adhesion of the connecting surface with another member such as a light emitting element via a layer of a solder or brazing material, so that a gap is easily formed therebetween, and the gap causes a decline in thermal conduction efficiency between the semiconductor element mounting substrate and another member.
Moreover, even if adhesion could be achieved without a gap therebetween, the recesses existing on the connecting surface remain at an interface therebetween as voids to hinder thermal conduction and still causes a decline in thermal conduction efficiency between the semiconductor element mounting substrate and another member, so that there is also a problem that neither case of these can take full advantage of favorable thermal conductivity of the diamond composite material. Therefore, the conventional semiconductor element mounting substrates described in Patent Documents 1 to 3 cannot sufficiently cope with a further higher power output by a light emitting element such as a semiconductor laser in recent years, and under the present circumstances, an effect to prevent the light emitting element or the like from causing a defective operation by heat generation of itself has come to be insufficient.
Moreover, the sizes of the recesses and protrusions have, dependent on the particle size of diamond particles, large depth and height of approximately 5 μm to 300 μm, while the thicknesses of the metal films formed on the connecting surface in Patent Documents 1 to 3 are considerably small in comparison therewith. For example, in Example 3 of Patent Document 1, the maximum thickness of an Ni film serving as the first metal film is set to 2 μm, and the thickness of an Au film serving as the second metal film is set to 0.2 μm, so that the total thickness is a mere 2.2 μm at a maximum. Moreover, in Example 3 of Patent Document 3, the thickness of an Ni film serving as the first metal film is set to 1 μm, and the thickness of a Pt film serving as the second metal film is set to 0.2 μm, so that the total thickness is a mere 1.2 μm. Therefore, there is also a problem that, even if the connecting surface is coated with the two layers of the metal films, it is impossible to fill and cover the recesses and protrusions therewith, that is, to fill the recesses with the metals to form the metal films so as not to remain as voids and to flatten the connecting surface with the recesses and protrusions buried in the metal films.
In Patent Document 4, it is described that a heat spreader serving as a semiconductor element mounting substrate is formed of the diamond composite material and a Cu material layer larger in thickness than the metal films explained in the foregoing is formed on a connecting surface of the heat spreader so as to fill and cover recesses and protrusions, to thereby provide the surface of the Cu material layer as a smooth surface. According to the configuration, the problems, explained in the foregoing, that unevenness formed on the connecting surface by the recesses and protrusions hinders adhesion of the connecting surface with another member such as alight emitting element, so that a gap is formed therebetween, and the recesses remain at an interface therebetween as voids to hinder thermal conduction, can be solved. Therefore, it becomes possible to mutually connect the connecting surface and another member without a gap therebetween via a layer of a solder or brazing material.    Patent Document 1: Japanese Unexamined Patent Publication No. 2003-309316 (Claims 1, 5, and 6, Paragraph 0014 to Paragraph 0015, Paragraph 0025 to Paragraph 0028, FIG. 2)    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-175626 (Claims 1, 6, 7, and 9, Paragraph 0018, Paragraph 0019)    Patent Document 3: Japanese Unexamined Patent Publication No. 2005-184021 (Claims 1, 2, and 4, Paragraph 0027 to Paragraph 0028, Paragraph 0050 to Paragraph 0051, FIG. 7)    Patent Document 4: Japanese Unexamined Patent Publication No. 2005-175006 (Claims 1 and 4, Paragraph 0009 to Paragraph 0011, Paragraph 0018, Paragraph 0020, Paragraph 0024, FIG. 1, FIG. 2)